The second law of thermodynamics is also known as the statement.

Kelvin-Planck statement or Clausius Statement.

Explanation:

The second law of thermodynamic deals with the direction of flow of energy which explains that the energy can only flow spontaneously from high grade to a lower grade energy i.e. only in the degrading direction of the energy.

It is supported by two statements,

Kelvin-Planck statement tells that it is impossible to construct a device working in a cycle which has no other effect than converting the heat energy from a reservoir into work output.

Clausius statement tells that it is impossible to construct a device operating in a cycle that produces no other effect than transferring heat from a low temperature reservoir to a high temperature reservoir.

Carnot's limit    

Explanation:

According to second law of thermodynamics, the entropy of a system  (isolated) cannot decrease with time and can remain constant only if all the processes involved are reversible ones. If the processes are irreversible, the entropy would increase.

There are two statements of second law:

1. Clausius statement:  

A device cannot be constructed which operates in a cycle whose effect is to just transfer heat from a body at low temperature to a body at high temperature.

2.  Kelvin-Planck statement:

A device cannot be constructed that operates in a cycle such that it with draws energy from a hot reservoir and completely delivers it in form of work.

3. dark matter

empty space could also be a simple vacuum